JP 2002-315193 A discloses an electric power control system that supplies electric power, which is stored in an electric power storage device, to a load arranged in a house. Specifically, the electric power control system includes an alternating direct converting unit arranged between the electric power storage device and an electric power supply system. Hereinafter, AC refers to alternating current, and DC refers to direct current. When an electric power source of the load is switched from the electric power supply system to the electric power storage device, the converting unit converts DC power stored in the electric power storage device to AC power. That is, when the electric power stored in the electric power storage device is discharged, the DC power stored in the electric power storage device is converted to the AC power. Then, the AC power is supplied to the load.
However, in the above-described electric power control system, when the AC power, which is discharged from the electric power storage device and converted by the converting unit, is supplied to the load, a phase of the AC power from the electric power storage device may be substantially different from a phase of the AC power supplied from the electric power supply system. When the AC power of the electric power storage device, which has the phase substantially different from the phase of the electric power from the electric power supply system, is directly supplied to the load, the AC power applied to the load sharply changes and a malfunction or a failure may occur to the load.